1. Field of the Invention
The invention relates to a radiotelephone installation for prepayment operation with security protection.
It is for general application in radio communications and more particularly in digital systems of communication with public mobile telephones operating in the 900 MHz band, also known as the Special Mobile Group (GSM), with autonomous telephone sets of the Pointel (registered trade mark) type, or with the mobile sets of the DECT (Digital European Cordless Telephone) type, etc.
2. Discussion of the Related Art
In general, such digital communications systems comprise:
at least one switched telephone network; PA1 an autonomous telephone subscriber access system comprising at least one base station linked to the switched telephone network, first means of authentication capable of generating an encryption function F with the aid of a secret key which is personal to each subscriber, and means of charging capable of calculating the call charges for the chargeable services of the switched telephone network; PA1 at least one autonomous telephone set comprising means of establishing radiotelephone intercommunication with the base station, second means of authentication capable of generating the encryption function F with the aid of the secret key which is personal to the autonomous telephone set subscriber, and means of prepayment capable of receiving and paying the call charges calculated and transmitted by the charging means.
It is currently expected that the autonomous telephone set will access a chargeable service of the switched telephone network following a confirmed authentication of the identity of the autonomous telephone set subscriber using the first and second means of authentication.
It should be noted that the function used for the authentication of the subscriber and the encryption function F may be identical, or similar, or completely different from each other, or may use different secret keys.
In practice, the authentication of the identity of the autonomous telephone set subscriber is an authentication of the active type with the exchange between the autonomous telephone set and the access system of a random number and of the transform of this random number by the encryption function F using the secret key which is personal to the subscriber.
This authentication of the subscriber makes it possible only to ensure the authenticity of the subscriber and the authorization for this subscriber to access the chargeable radiotelephone services.
It does not enable the system engaged in the dialogue with the subscriber identified and authorized for access in this way to ensure the integrity of the means of prepayment receiving the call charges throughout the call.
Operators of such radio communications systems have observed that an abuser may create a blockage between the receiving prepayment means and the transmitting charging means on radio links which are already established, since the radio medium does not provide any physical protection.
Moreover, an abuser may fraudulently simulate, by appropriate means of processing, the entry of charging units in the receiving prepayment means (such as a prepayment card) since the means of prepayment which are used with the autonomous telephone set are not subject to any physical check of security or integrity by the operator.
Consequently, the current prepayment cards are not satisfactory for providing a secure prepayment service in a radiotelephone installation, particularly when the autonomous telephone set is located in a hired automobile which is not subject to any check by the hiring company.